How can I apply some material included in a node-pack library?
Those are Node groups. Node groups are node setups can take input to give an output with which you can use in other nodes. (Think of it like making a new node of many nodes put together, similar to a function in computer programming.) This also allows you to create a node setup then use it in many places.
The collection of node groups you linked to is made up of Shader nodes and Compositing nodes. The author of the collection you linked to also explains the function of each node group here.
Adding a node group to your blendfile:
You can use them in your own file Appending them. You can do this by pressing ShiftF1&gt; append, then navigating to the blendfile with the nodegroups which you want to use:

Pick the type of data to append, in this case we want Node groups so pick NodeTree:

Select the node group you want to import into your file:

See this question for more information on appending and linking.
You can now add this node group to your own node tree.
In the Node editor, press ShiftA&gt; Add node &gt; Group and select the appended node group:

You now have a new &quot;node&quot; which you can use in your node tree the same way you would use any normal node:

In the case of this node group, it supplies a Shader output, however some node groups return a value or an image (or a vector, or even multiple different outputs).
Editing node groups:

You can edit the nodes in a node group by selecting it and pressing Tab.

You can edit, add and remove inputs and outputs of the node group in the Properties region (N):




You can have node groups inside node groups, so to go to the parent group (the node tree in which the currently active group is located) press the  button on the header. (you can think of this behavior as a file tree with the base node editor as root)

You can ungroup nodes by selecting the group node and pressing AltG. This will place all the nodes in that node group in the current node tree. (I.e. the currently open group or the base node editor)


Other things to note:
The way node groups look and function changed a lot in 2.67. See the wiki for a full description of the changes